Group-13-nitride-based semiconductor materials, such as gallium nitride (GaN), are known as materials used for semiconductor devices, such as blue light-emitting diodes (LEDs), white LEDs, and laser diodes (LDs). There have been developed vapor phase epitaxy and liquid phase epitaxy as methods for manufacturing a group 13 nitride crystal.
The vapor phase epitaxy is a method for growing a group 13 nitride crystal on a seed substrate in a vapor phase. Examples of the vapor phase epitaxy include metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). To manufacture a group 13 nitride crystal self-standing substrate by the vapor phase epitaxy, a method for reducing the dislocation density is frequently used, such as epitaxial lateral overgrowth (ELO). In the method, GaN is thickly grown by HVPE on a heterogeneous substrate, such as a sapphire substrate and a gallium arsenide (GaAs) substrate, and the GaN crystal thick film is then separated from the heterogeneous substrate, for example. The GaN self-standing substrate manufactured in this manner has a dislocation density of approximately 106 cm−2, for example.
The liquid phase epitaxy is a method for growing a group 13 nitride crystal on a seed substrate (a seed crystal) in a liquid phase. Examples of the known liquid phase epitaxy include a flux method. The flux method is a method for growing a group 13 nitride crystal on a seed crystal by: arranging a seed crystal made of a group 13 nitride crystal in a mixed melt containing an alkali metal, such as sodium (Na) and potassium (K), and a group 13 element, such as Ga and aluminum (Al); and supplying nitrogen to the mixed melt. In the flux method, the mixed melt is heated to approximately 900° C. under an atmosphere at nitrogen pressure equal to or lower than 10 MPa to dissolve nitrogen into the mixed melt from the vapor phase. Thus, the nitrogen reacts with the group 13 element in the mixed melt, thereby growing the group 13 nitride crystal. The flux method enables the crystal to grow at lower temperature and lower pressure than other types of liquid phase epitaxy do. Furthermore, the grown group 13 nitride crystal advantageously has a dislocation density lower than 106 cm−2, for example.
Patent Literatures 1 to 3 describe a method for growing a GaN crystal on a seed crystal made of a GaN substrate by the flux method. Patent Literature 3 describes an off-angle of a seed substrate used in the flux method. Patent Literatures 4 and 5 describe a reaction vessel used in the flux method. Patent Literature 4 describes that, by using Al2O3 as a material for the reaction vessel, Al2O3 is dissolved during the crystal growth process, thereby changing the weight of the reaction vessel. Patent Literature 6 describes a result of measurement with cathode luminescence of a GaN crystal grown by the vapor phase epitaxy. Non-patent Literature 1 describes luminescence characteristics in a case where impurities are included in a GaN crystal.
To meet recent demands for reduction in cost of white LEDs and application of white LEDs to electronic devices, for example, a group 13 nitride crystal is expected to have a larger diameter. A relatively small group 13 nitride crystal self-standing substrate can be usually manufactured at lower cost by the vapor phase epitaxy than by the flux method. In a case where the self-standing substrate is enlarged and the vapor phase epitaxy is applied, however, failures such as a warp and a crack easily occur because of a difference in the coefficient of thermal expansion and a difference in the lattice constant between the heterogeneous substrate and the group 13 nitride crystal, for example. Thus, it is difficult to manufacture a large and high-quality group 13 nitride crystal by the vapor phase epitaxy alone.
Because the flux method requires no heterogeneous substrate, a failure such as a warp does not easily occur. Thus, the flux method is suitably used to manufacture a large and high-quality group 13 nitride crystal. The flux method, however, requires a higher manufacturing cost than that of the vapor phase epitaxy.
In view of the circumstances described above, the present invention aims to manufacture a high-quality group 13 nitride crystal at low cost.